Systems for controlling the output torque of an internal combustion engine are generally known and extensively used, particularly in the diesel engine industry. One example of such a control system is set forth in U.S. Pat. No. 5,703,325 to Bellinger, which is assigned to the assignee of the present invention, and which is concerned with protecting drivetrain components by limiting engine output torque so as not to exceed the input torque capacities of the various drivetrain components. However, while the concepts disclosed in the Bellinger patent are useful in protecting drivetrain components during so-called positive engine output torque conditions (i.e. positive engine fueling conditions), they do not address protecting such drivetrain components during coast mode operation (i.e. zero engine fueling conditions) when the engine retarding torque, due to engine compression brake operation, may exceed the torque capacities of such components.
One particular concern during coast mode operation is the torque capacity of the vehicle axle. An example of a known axle 10 is illustrated in FIGS. 1 and 2, wherein the axle 10 includes a so-called universal joint 14 connected to a shaft 16 of a pinion gear 18. Pinion gear 18 is adapted to mesh with a ring gear 20, wherein the ring gear 20 is connected to axle shafts 22a and 22b which are, in turn, coupled to wheels 12a and 12b respectively. Under positive engine output torque (drive mode) operation, a propeller shaft (not shown) extending from a transmission or other drivetrain component drives the universal joint 14 which causes the pinion gear to rotate in the direction shown by arrow 32 in FIG. 2. The teeth 28 and grooves 30 therebetween of gear 18 mesh with corresponding teeth 24 and grooves 26 therebetween of gear 20 so that pinion gear 18 is operable to drive ring gear 20 in the direction shown by arrow 34 of FIG. 2. Ring gear 20 is coupled to axle shafts 22a and 22b, and thereby drives wheels 12a and 12b via shafts 22a and 22b under positive engine output torque (drive mode) operation. During coast mode operation, the wheels 12a and 12b drive axle shafts 22a and 22b respectively, thereby driving ring gear 20. The ring gear 20, in turn, drives the pinion gear 18 which, in turn, drives the universal joint 14 against the engine retarding torque provided by engine compression brake operation during zero fueling (coast down). The engine retarding torque seen by the axle 10 is the actual engine retarding torque multiplied by the currently engaged gear ratio of the transmission (not shown).
The teeth 28 and grooves 30 of pinion gear 18, and the teeth 24 and grooves 26 of ring gear 20, are typically designed for optimum performance and durability under drive mode operation (positive engine torque). Therefor, by design, the input torque capacity of axle 10 typically greater in drive mode operation than in coast mode operation. As a result, while axle 10 may have sufficient input torque capacity to withstand the maximum output torque of the engine, it may not have sufficient torque capacity to withstand maximum engine retarding torque levels. Under such conditions, damage to the teeth 28 of pinion gear 18 and/or teeth 24 of ring gear 20 results, thereby compromising the reliability and durability thereof.
What is therefore needed is a system for controlling engine retarding torque during coast mode operation so that the engine retarding torque levels do not exceed the coast mode torque capacities of the various drivetrain components in general, and of the one or more vehicle axles in particular.